In general, a construction machine such as an excavator in the prior art adopts a hydraulic driving system in which a hydraulic pump is driven by a fuel engine and an actuator is driven by hydraulic pressure. Such a conventional excavator performs not only work that requires the maximum output power but also work that requires an output power lower than the maximum output power, for example, about 80% or 50% of the maximum output power. In the case of performing work with such a low output power, the engine efficiency and the fuel efficiency deteriorate. FIG. 1 is a necessary torque output relationship diagram illustrating an excavator system according to the prior art. As shown in FIG. 1, in a conventional hydraulic excavator, an opening area of a main control valve (MCV) 7 is adjusted in accordance with an output of a pilot valve 6 through a user's manipulation of a lever 5.
Hydraulic fluid discharged from a hydraulic pump 13 is transferred to respective hydraulic systems with its flow rate adjusted in accordance with the opening area of the MCV 7.
In such a conventional hydraulic excavator, it is sometimes required to perform work while greatly changing the output power of an engine so as to cope with a great load change, and it is required to seek the improvement of the engine's fuel efficiency by effectively utilizing the engine output. Accordingly, a hybrid technology using an electric generator that is adopted in a hybrid vehicle has been planned to be applied to an excavator. That is, a hybrid excavator has been proposed in which an electric generator is connected to an engine. According to the hybrid excavator, in the case of performing a light-load operation, the electric generator is operated to generate electricity to charge a battery using a portion of the engine output, while in the case of performing a swing operation or using another electric device, electric energy is supplied from the battery to drive the actuator.
However, the excavator has the characteristics of an engine output power, a torque/speed operation region, and a power transfer device, which are quite different from those of a hybrid vehicle, and thus it is not easy to apply the hybrid technology to the excavator. According to a control system for a hybrid excavator proposed in the prior art, in which the drive torque of the hydraulic pump and the output torque of the engine are simply compared with each other, such that the motor-generator generates power to charge a battery using a surplus engine output torque in the case of performing the light-load operation, whereas in the case of performing the heavy-load operation, electric energy charged in the battery is supplied to the motor-generator to operate the motor-generator as a motor so that the engine is supplemented as much as the shortage of the engine output torque, it is difficult to seek improvement of the fuel efficiency by effectively utilizing the output of the engine of the excavator which has the characteristics of working region of diverse loads, engine output, torque/speed operation region, and power transfer device which are quite different from those of the hybrid vehicle.
In particular, since the excavator performs various kinds of work, in which working loads are different from one another, such as excavating, dipping-out, leveling, pounding, and the like, the transmission technology for the hybrid vehicle is not adopted in the hybrid excavator in the prior art, and thus it is very difficult to optimize the fuel efficiency.
Further, in the excavator, if the relationship between the kind (or mode) of the work and the engine output or the battery charging state is not taken into consideration, problems, such as energy waste, deterioration of working efficiency, and the like, may occur.